(Not Applicable)
(Not Applicable)
1. Field of the Invention
This invention relates generally to the field of semiconductor wafer fabrication, and more particularly to the field of chemical mechanical planarization (CMP) of thin films used in semiconductor wafer fabrication.
2. Description of the Related Art
The production of integrated circuits begins with the creation of high quality semiconductor wafers. A semiconductor wafer typically includes a substrate, such as a silicon or gallium arsenide wafer, on which a plurality of transistors have been formed. Transistors are chemically and physically formed in and on a substrate by patterning regions in the substrate and patterning layers on the substrate. The transistors are interconnected through the use of well known multilevel interconnects to form functional circuits. Typical multilevel interconnects are comprised of stacked thin films, commonly comprised of one or more of the following: titanium (Ti), titanium nitrite (TiN), tantalum (Ta), aluminum-copper (Alxe2x80x94Cu), aluminum-silicon (Alxe2x80x94Si), copper (Cu), and tungsten (W).
During the wafer fabrication process, the wafers may undergo multiple masking, etching, dielectric deposition, and conductor deposition processes. An extremely flat, or planarized, surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. In general, a wafer can be polished to remove high topography, surface defects such as crystal lattice damage, scratches, roughness or embedded particles. As the size of integrated circuits continues to decrease and the density of microstructures on an integrated circuit continues to increase, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish the surface of a wafer in order to obtain the most planarized surface possible.
CMP is routinely used to planarize the surface of the layers, or thin films, of the wafer during the various stages of device fabrication. CMP has emerged as the planarization method of choice because of its ability to planarize better than traditional planarization methods. During a CMP process, polishing planarizes surfaces to very precise tolerances, which is essential for maintaining the precise photolithographic depth of focus required for integrated circuit chip fabrication. In a typical CMP process, the wafer is held by a rotating carrier with the active wafer surface facing a rotating polishing table, called a platen. On top of the platen is a porous polyurethane polishing surface on which is poured a slurry. The slurry can be colloidal silica suspended in an aqueous solution. Slurries with different chemical compositions are used to polish metals and other films. During metal polishing, the slurry chemically reacts with the wafer""s surface, forming a passive layer on a portion of the wafer""s surface, while the mechanical force exerted by the pad and the colloidal silica particles abrades the wafer""s surface, removing the passive layer.
A CMP slurry serves several functions. Most notably, it is the medium in which abrasive particles are dispersed. Additionally, it furnishes the chemical agents which promote the chemical process. To obtain optimum results from CMP processing, there must be a synergistic relationship between the chemical and mechanical processes.
For example, CMP slurries for polishing a metal layer commonly comprise a metal oxidizer and an abrasive agent. The oxidizer reacts with the metal to form a passive metal oxide layer. During the polishing process, the abrasive agent removes the passive oxide layer from elevated portions of the metal layer. Depressed portions of the metal layer surface are not subjected to mechanical abrasion and, therefore, the protected material underlying depressed portions of the passive oxide layer is not polished. This process continues until the elevated portions of the metal layer have been polished away, resulting in planarization.
The ideal polishing process can be described by Preston""s equation: R=Kp*P*V, where R is the removal rate, P is the applied pressure between the wafer and the polishing surface, V is the relative velocity between the wafer and the polishing surface, and Kp is a function of consumables such as polishing surface roughness, elasticity, and chemistry. The ideal CMP process has constant pressure between the polishing surface and the wafer, constant polishing surface roughness, elasticity, area, and abrasion effects, and constant velocity over the entire wafer surface. Having constant velocities at points which are distant from the center of the wafer is generally preferable to having fluctuating velocities because the removal rate is much easier to control when constant velocity conditions exist. For example, when points at a distance from the center of the wafer are exposed to alternating high and low velocities, the abrasive material may scratch the surface of the wafer and result in a non-planarized surface. Non-uniform removal of films from the surface of a wafer is a common problem encountered during CMP processing because there are numerous variables which can affect planarization.
In a typical CMP process, the wafer carrier and the platen rotate in the same direction, but with the two rotating axes offset by some distance. This arrangement results in relative linear motion between any position on the wafer and the polishing surface. Thus, removal caused by the polishing surface is related to the radial position of the wafer relative to the platen. The removal rate increases as the wafer moves radially, or linearly, outward relative to the platen. Removal rates tend to be higher at the edges of the wafer than at the center of the wafer. As a result, wafer surfaces tend to become higher at the center of the wafer as compared to the edges of the wafer. Reducing this center-to-edge variation results in a more planarized wafer surface.
Attempts have been made to reduce this center-to-edge variation by polishing in non-linear polishing patterns. One approach includes affixing a mechanical template having a non-linear opening to a polishing surface. A rotating motor moves a wafer carrier along the edges of the non-linear template, allowing the wafer carrier to traverse the surface of the polishing surface in a non-linear manner. This approach is significantly limited, however, because it requires attaching a device to the polishing surface. Such a configuration can significantly reduce the polishing surface lifespan by causing uneven wear of the polishing surface. The direct contact between the template and the polishing surface also reduces the lifespan of the polishing surface because the template can introduce particles and other defects into the polishing surface. Another approach involves the use of a non-linear carrier displacement mechanism for moving a wafer carrier across a polishing surface. A drawback to this configuration is that it does not provide a means for moving a wafer across a polishing surface along a substantially figure eight path.
The present invention relates to an improved apparatus and method for planarizing the surface of a substrate, such as a semiconductor wafer. In one embodiment of the invention, the apparatus for polishing substrate surfaces includes a polishing surface, a holding device for holding a substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and structure for moving the holding device in a substantially figure eight path relative to the polishing surface. The moving structure can comprise a motor and an actuating arm connecting the motor to the holding device.
In another embodiment, the apparatus for polishing substrate surfaces includes a polishing surface, a holding device for holding a substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and a structure for moving the holding device in a substantially curvilinear path relative to the polishing surface. In this embodiment, the moving structure can include a drive which is attached to the holding device, structure for rotating the drive, and at least one steering device for steering the drive in a substantially curvilinear path relative to the polishing surface. The substantially curvilinear path can be a substantially figure eight path. The steering device can be one or more cams.
Yet another embodiment of the invention comprises a polishing surface, a holding device for holding at least one substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and a moving structure. The moving structure can include a drive which is attached to the holding device and rotates the holding device, and counter-rotating devices having structure for engaging the holding device. The counter-rotating devices alternately engage the holding device, thereby moving the holding device in a substantially curvilinear path relative to the polishing surface. The substantially curvilinear path can be a substantially figure eight path.
A method for polishing substrate surfaces according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, and moving the holding device in a substantially figure eight path relative to the polishing surface with moving structure. The step of moving the holding device in a substantially figure eight path relative to the polishing surface can be performed with a motor and an actuating arm connecting the motor to the holding device.
Another method according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, rotating the holding device with a drive attached to the holding device, and steering the drive in a substantially curvilinear path relative to the polishing surface with at least one steering device. The substantially curvilinear path can be a substantially figure eight path, and the steering device can be one or more cams.
Still another method according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, rotating the holding device with a drive attached to the holding device, providing a plurality of counter-rotating devices having structure for engaging the holding device, and rotating the counter-rotating devices. The counter-rotating devices alternately engage the holding device, whereby the holding device moves in a substantially curvilinear path relative to the polishing surface.